Make an adapter out of the disc itself. Mark 8 or 16 holes around the sprocket at even points. Then drill the holes. Get 1/2 inch spacers with high tensile nuts and bolts.
The trick is to use the wheel to get the markings exact. I used strong paper clamps to hold the disc and sprocket together, then spun the wheel. By eye, I was able to see if it was spinning off center and therefore could make adjustments. Lucky for me, I got it 1st go.
Mind you, you will have to cut out the center of your sprocket to clear the 6 holes on the hub that the disc brake lines up to.
Can post pics up later on.